1. Field of the Invention
The present invention relates to a device for determining flow parameters, particularly the temperature, the flow velocity and its changes, in a stream of fluid to be monitored, particularly in smoke and gas intake detectors, having a thermoelectric air stream sensor that is operated in a constant temperature mode, a thermoelectric temperature sensor, and a regulation circuit for setting an excess temperature ΔT at the air stream sensor, as well as to a method for operating such a device, a corresponding work method, and a fire recognition device or oxygen measurement device equipped with such a device.
Devices for determining flow parameters, of the type stated initially, as well as corresponding methods for operating such devices, are particularly known from heating wire anemometry. In this connection, a heated wire is introduced into a flowing fluid; information about various flow parameters can be obtained using the amount of heat taken from the fluid.
In the case of heating wire anemometry, there are two fundamental modes of operation: constant flow operation and constant temperature operation, which is used in most cases, since here, among other things, the thermal inertia of the heating wire (sensor) is circumvented and thereby greater accuracy of the sensor is achieved.
The fundamental idea of the constant temperature operating mode is that of reducing the influence of the thermal inertia of the sensor in that the heating wire is kept at a constant temperature (resistance) and the heating current required for this is used as a measure of the flow velocity of the fluid. For this purpose, a Wheatstone bridge circuit is generally used, whereby the resistance and therefore the temperature of the heating wire always has a constant value, by means of feedback. In thermal equilibrium, the heat loss of the sensor must be equal to the added electric power. From the point of view of anemometry, the relationship between the fluid velocity and the electric power is of primary interest. This relationship is extremely complex, non-linear, and can only be described by means of an empirical law (King) that must be modified in accordance with the given circumstances, in each instance. In the assessment, the use of a linearizer therefore becomes necessary.
2. Description of Related Art
FIG. 1 shows the fundamental schematic of a constant temperature anemometer. In the state of equilibrium, a certain voltage is applied at the perpendicular diagonal C-D of the bridge, which voltage is supplied by a servo amplifier 15. If the convective cooling at the sensor 18 changes, then a small voltage will occur at the horizontal diagonal A-B, which is fed back to the perpendicular diagonal C-D of the bridge, after having been amplified many times. In this connection, the polarity of this feedback voltage is selected in such a manner that the bridge equalizes automatically.
Aside from the complex relationship between the fluid velocity and the electric power detected as the measurement parameter, there is an additional problem in that the sensor responds to any change in heat removal, which can also be caused by a change in the temperature or the pressure of the flow medium, for example. This is particularly problematic if the method is being used continuously, in order to be able to draw reliable conclusions with regard to the status of the pipe system in which the fluid is flowing, for example, on the basis of changes in the measured flow parameters.
Particularly when monitoring the air stream in intake pipe systems in aspirative fire recognition devices or oxygen measurement devices, it is important that a blockage or a pipe break in the intake pipe system can be reliably detected, in order to be able to guarantee error-free operation of the fire recognition device or oxygen measurement device. Here, an aspirative fire recognition device is understood to be a device that actively draws in a representative partial amount of the space air of a space to be monitored, at a plurality of locations, by way of a pipeline system or channel system, and then passes these partial amounts to a detector for determining a fire characteristic value, or for detecting gases in the air, particularly oxygen.
An aspirative fire recognition device essentially consists of an intake pipe system having individual, small openings, a fan that draws an air sample from the target space via the intake openings of the intake pipe system, as well as a detector in which fire characteristic values of the air sample drawn in are subsequently determined. Since an aspirative fire recognition device actively draws in air samples from the target space, and therefore draws in any fire characteristic values that might be present, such devices react to fires that are starting in much faster and more sensitive manner than traditional solutions. Therefore the best possible intervention possibility is presented.
The term fire characteristic value is understood to mean physical variables that are subject to measurable changes in the environment of a fire that is starting, e.g. the ambient temperature, the proportion of solids or liquids or gases in the ambient air (formation of smoke in the form of particles or aerosols or vapor), or the ambient radiation. An aspirative fire recognition device is furthermore used anywhere where even the smallest, barely perceptible fire recognition characteristic values are supposed to be detected, and particularly serve to monitor premises and spaces, for example those containing EDP systems or server rooms.
In closed spaces whose furnishings and fixtures react sensitively to the effects of water, such as EDP areas, electrical switching or distributor rooms, or storage areas with high-value goods, so-called inertiatization methods are increasingly being used to reduce the risk of fire and for extinguishing fires. The extinguishing effect that results from this method is based on the principle of oxygen displacement. Normal ambient air is known to be comprised of 21 vol.-% oxygen, 78 vol.-% nitrogen, and 1 vol.-% other gases. For extinguishing and preventing fires, the inert gas concentration in the space in question is increased, and the oxygen proportion is decreased, by introducing an oxygen-displacing inert gas, such as pure nitrogen. Many substances no longer burn if the oxygen proportion drops below 15–18 vol.-%. Depending on the flammable material present in the space in question, a further decrease in the oxygen proportion, to 12 vol.-%, for example, might be necessary.
Such an inert gas device for carrying out the stated inertiatization method has essentially the following components: an oxygen measurement device to measure the oxygen content in the target space to be monitored; a fire recognition device for detecting a fire characteristic value in the space air or the target space; a control for evaluating the data of the oxygen measurement device and the fire recognition detector, and for sequence control of the inertiatization method; and a system for the production and sudden introduction of inert gas into the target space.
The oxygen measurement device serves to set the base inertiatization level in the target space. If a threshold of the oxygen concentration is exceeded (for example due to a leak in the target space), the control issues a command to a special system for introducing inert gas into the space, so that the oxygen proportion is reduced. The oxygen measurement device signals when the threshold value of the base inertiatization level has been reached again. In this connection, the location of the base inertiatization level is dependent on properties of the space.
In a preferred use, an aspirative fire recognition system is combined with an inert gas device for preventing and/or extinguishing fires. In this connection, the oxygen measurement device and the fire recognition device of the inert gas device are integrated into the aspirative fire recognition system. The latter then takes on the task of making the data required for monitoring the target space available to the control, from the air sample drawn in.
In order to be able to guarantee problem-free and, to the greatest extent possible, maintenance-free functioning of an aspirative device, it is necessary to continuously monitor the volume stream of the air sample being supplied to the detector. However, the volume stream is dependent on the mass stream and the density of the air sample added, and this in turn is a function of the air pressure and the temperature. Therefore monitoring of the volume stream proves to be a complicated task in terms of measurement technology. In order to furthermore be able to reliably detect blockages in or damage to the intake pipe system, i.e. the intake openings, a high degree of measurement accuracy with regard to the volume stream monitoring is required. This also includes, among other things, compensation of the influence of the air density, i.e. the air pressure, in the case of the measurement technology being used to monitor the volume stream.